Front seat back frames for a vehicle are required not only to have rigidity to appropriately support an occupant's body but also to have high strength to withstand large load produced by acceleration exerted on the occupant when the vehicle collides.
Seat back frames generally have a structure in which members obtained by bending metal pipes or members obtained by performing sheet metal working on metal sheets are joined to each other by weld joining. However, due to demands for lighter vehicle weight, there have been demands for lighter seat back frames as well. In view of this, manufacturing a seat back frame with a resin or a fiber-reinforced composite material has been considered.
When a resin or a fiber-reinforced composite material is used for a seat back frame, how to attach a reclining device is an important issue: A reclining device is generally formed of complicated parts such as a gear to fix a seat back frame at a given angle in order for an occupant to have an appropriate seating posture, and manufactured using metal because the reclining device needs strength high enough to withstand moment exerted on the seat back frame when the vehicle collides. In the case of a metal seat back frame as mentioned above, the reclining device can be joined directly to the seat back frame by weld joining. However, in the case of using a resin or a fiber-reinforced composite material for a seat back frame, a design that allows the reclining device to be securely fixed to the seat back frame is necessary.
Patent Literature 1 discloses a seat back frame which is formed of molded resin parts and in which each of side frames has a′plate-shaped metal attachment bracket attached to a lower end portion thereof and is connected to a hinge mechanism (reclining device) through this bracket. In the case of this structure, when a large moment load exerted on the seat back frame during collision of the vehicle is transmitted to the reclining device, the load is highly likely to be concentrated at the joining portion of the resin and the bracket. For this reason, it is concerned that both the resin frame and the metal plate bracket may need to be large in size. In addition, attaching the metal plate bracket to the molded resin part requires a post-working such as thermal caulking.
Patent Literature 2 discloses a seat back frame which is formed as a single integral member made of a fiber-reinforced composite material containing a thermoplastic resin matrix and reinforcing fibers, and in which a hinge as part of a reclining device is over-molded in the single integral member. Here, a structure in which the hinge is attached to either one of the left and right sides is shown as a first embodiment. In this case, it is only the hinge mechanism on the one side that will support large moment load exerted on the seat back frame when the vehicle collides. For this reason, it is concerned that the hinge and the seat back frame may need to be large in size. A structure in which the hinge is attached by over-molding on both left and right sides is shown as a second embodiment. In this case, it is possible that the axes of the left and right hinges may be misaligned from each other, thereby impairing smooth motions of the seat back frame, due to contraction or warpage of the fiber-reinforced composite material after its molding. Further, in either embodiment, the shape of the hinge is extremely complicated, and therefore the weight and cost are unlikely to be reduced. Furthermore, the hinge is over-molded while expanding the molded part at inner pressure, and therefore sufficient joining strength is unlikely to be achieved.